The present invention is directed to an aqueous catalyst composition and method of depositing an ultra-thin metal or metal alloy layer on a substrate. More specifically, the present invention is directed to an aqueous catalyst composition and method of depositing an ultra-thin metal or metal alloy layer on a substrate where the aqueous catalyst has high surface area particles.
Many industries where workers desire to coat or form one or more metal or metal alloy layers on substrates employ catalysts. Such catalysts are employed in electroless deposition of metal or metal alloys. Electroless deposition or plating is based on the presence of a chemical reducing agent being added to the deposition bath. Such chemicals supply electrons to substrate metals, which transmit the electrons to the positively charged metal ions in the bath reducing these ions to metal in the same manner in which electric current reduces metal ions to metals in electrolytic or electrodeposition baths.
Electroless plating produces several desirable results. Workers often have difficulty in depositing metal layers of uniform thickness on substrates with crevices or holes using electrolytic methods of plating. This attribute is important in many industries such as in the electronics industry, in which printed circuit or printed wiring boards demand uniform metal deposits plated into high aspect-ratio through-holes. Other properties and applications of electroless plating are deposits which may be produced directly upon nonconductors, deposits in which are often less porous than electrolytic plating, and also deposits which often have unconventional chemical, mechanical or magnetic properties (such as higher hardness and wear resistance).
Another attribute of electroless plating is that the process is auto-catalytic and deposition occurs on a catalytic surface. Accordingly, a catalyst is required. Catalysts employed in electroless metal deposition vary widely in composition depending on the metal or metal alloy to be deposited as well as the use of the article made. In addition to the manufacture of printed wiring boards, electroless plating using catalysts are employed in the manufacture of various decorative articles, and in numerous other electronic applications such as in the formation of electromagnetic interference (EMI) and radio frequency interference (RFI) shielding.
EMI radiation is created by operation of many diverse forms of electronic equipment ranging from microwave equipment to home computers. The radiation occurs because electronic devices emit “noise” in a frequency range of 60 Hz to more than 1000 MHz, and is picked up by other devices or by conduction through power lines that act as antennas. EMI radiation may interfere with other devices and has been known to cause such diverse problems as interference with police mobile radios, communication systems, scientific test equipment and cardiac pacemakers.
One approach to limiting electromagnetic containment is the use of an EMI shield to contain the radiation. Containment requires special shielding materials, components, and structures, which prevent generated energy from escaping and acting as a source of disturbance.
Effectiveness of electromagnetic containment is determined by the degree to which the field strength is attenuated as a result of reflection or absorption by the shielding material. Shielding efficiency is calculated as a logarithmic function of the ratio of unshielded EMI transmission to shielded EMI transmission and is expressed in decibels (db). Because of its logarithmic nature, an increase of 30 db in shielding efficiency for a given wavelength or frequency of electromagnetic radiation represents a 1000% increase in the shielding efficiency of a coating. A coating with a shielding efficiency of 30 db, for example, eliminates 99.9% of the total EMI radiation. A 60 db coating eliminates 99.9999% of the total EMI radiation.
A number of different shielding methods have been used commercially. One method involves applying a metallic coating over a plastic housing for electronic devices. Such methods include galvanic deposition, spray coating such as by arc-spraying or spraying the metal on as a paint, cathode sputtering, chemical metallizing and vacuum metallizing. Metal coatings have included copper, silver, chromium, nickel, gold and zinc. Such methods have suffered from a number of deficiencies such as macro or microscopic cracking, peeling of coatings, limited shielding effectiveness, oxidation of metals in the coatings, distortion of thermoplastic substrates, and expensive application equipment.
A more suitable method of forming an EMI shield has been by electroless deposition of a metal on the non-conductive housing materials. Electroless deposition of non-conductors such as plastics involved immersing a part in a series of aqueous baths, which both prepare the surface of the part for deposition and permit metallization. Following conventional pretreatment steps, the part is then immersed into a catalyst containing noble metals, such as a colloidal tin/palladium catalyst, to render non-conductive surfaces catalytic to deposition of the desired plating metal. Following catalysis, the part is then immersed into an electroless plating bath containing dissolved metals which, in contact with the plating catalyst, results in deposition of a coating of the metal onto the catalyzed surface.
While the foregoing electroless catalyst and method was superior to many of the earlier methods employed to address the problem of EMI shielding, the electroless coating process was not selective. The entire part was immersed into the colloidal catalyst solution followed by immersing the part into a metal plating solution. The result was that metal was plated over the exterior as well as the interior surface of the non-conductor part. Where aesthetics are important in the marketing of electronic components, an exterior metal coated housing for the electronic component is undesirable. Typically, the industry paints the housing. This is a time consuming and wasteful step, especially where housings are most often molded in a desired color. Accordingly, the industry developed an improved method of selectively depositing a metal on a non-conductive substrate.
U.S. Pat. No. 5,076,841 discloses a method of selectively depositing a metal on a non-conductor by an electroless method. The catalyst is sprayed or painted on the part of the non-conductor where metallization is desired. Portions of the non-conductor where metallization is undesired are masked prior to application of the catalyst. The catalyzed or primed non-conductor is then immersed into a suitable electroless metal plating bath. Metal or a metal alloy is deposited on the selective sites of the non-conductor where the catalyst was applied.
The catalyst of the '841 patent is a hydrous oxide of a noble metal such as silver oxide, palladium oxide, and platinum oxide. A hydrous oxide of copper also may be employed. The hydrous oxide of the metal is deposited as a colloid on inert, irregularly shaped, colloidal carrier particles such as carbon, various types of silicas including synthetic calcined silicas, synthetic precipitated silicas, silicas of fossil origin (diatomaceous), detritic natural silicas (powdered or micronized sand); alumina; and pigments such as titanium dioxide. The colloidal particles are irregularly shaped and have jagged edges to penetrate a substrate surface. Carrier particles range from between about 0.1 and 500 microns and have a surface area range of between 100 and 900 m2/gm. The catalyst may be aqueous based or organic solvent based. Various film-forming resins are included in the catalyst compositions.
In addition the catalytic compositions include an organic solvent to solvate or condition a substrate on which the catalyst is applied to promote bonding or adhesion of the catalyst to the substrate. Solvation of the substrate permits penetration of the substrate by the catalyst, however, such solvation may lead to surface defects in the substrate. Solvation results in a roughened surface such that metal layers deposited on the substrate form a lock and key bond. The irregular surface may result in a non-uniform metal layer, which may result in non-uniform shielding. Additionally, using solvent swells present a hazard to workers and the environment because many solvents are toxic, carcinogenic and require special and costly disposal procedures. Examples of such solvents include acetone, methyl ethyl ketone, toluene, isopropyl alcohol, ethers and ether acetate and propylene glycol alkyl ether acetate. Accordingly, there is a need for an improved composition and method of forming a metal layer on a non-conductive substrate.